Fluid system

ABSTRACT

A fluid system having a variable displacement pump fluidly connected through valving means to a fluid linear actuator in such a manner that fluid pressure directed to one portion of the linear actuator causes expansion thereof at a selected rate, while fluid pressure directed to another portion of the linear actuator causes contraction thereof at the same rate. The fluid linear actuator comprises a main fluid cylinder having a piston reciprocably mounted therein, with a piston rod extending from one side of the piston and externally of the main fluid cylinder. The piston divides the main fluid cylinder into expansible pressure chambers, one of which is exposed to the rod carrying side of the piston to generate a force thereagainst when pressurized to move the piston in one direction, while the opposite side of the piston is exposed to the pressure of the fluid in the other pressure chamber which exerts a force to move the piston in an opposite direction. A secondary fluid cylinder is carried by the main fluid cylinder with the longitudinal axes of the secondary and main fluid cylinders being radially spaced and parallel to one another. The secondary cylinder has a piston reciprocably mounted therein forming one pressure chamber on the side of the piston in which a second piston rod is mounted and which piston rod extends from the secondary cylinder for connection with the piston rod of the main fluid cylinder, such that the main cylinder piston and the secondary piston reciprocate together as a unit. The effective pressure responsive area of the side of the piston in the main cylinder opposite the rod carrying side thereof is substantially equal to the sum of the effective pressure responsive area of the rod side of the piston in the main cylinder and the effective pressure responsive area of the rod side of the piston within the secondary cylinder, such that the force generated against the two pistons for moving the pistons in one direction is equal to the force exerted against the pistons for moving the same in the opposite direction. Valving means are provided for interconnecting the pressure chambers associated with both the main and secondary fluid cylinders so that the rate of expansion and contraction of the cylinders may be selectively increased as the force exerted on the pistons is decreased and vice versa to provide a fluid linear actuator having a wide speed-force range.

United States Patent 1 Sclreafer [451 Feb. 19,1197l FLUID SYSTEM [76]Inventor: Robert E.- Scheafer, 3325 Newgate Rd., Troy, Mich. 48084 [22]Filed: Feb. 17, 1972 [21] Appl. No.: 227,057

[52] U.S. Cl. 91/411 B, 60/97 H, 60/425 [51] Int. Cl. FlSb 11/16 [58]Field of Search. 60/52 R, 97 H, 425; 91/411 B, 91/411 R [56] ReferencesCited UNITED STATES PATENTS 1,843,082 1/1932 Ferris et al 60/97 H1,866,348 7/1932 Ferris 60/97 H 2,497,608 2/1950 Herrstrum et al 60/52 R2,935,852 5/1960 Russell 91/172 Primary Examiner-Edgar W. GeogheganAttorney, Agent, or FirmHauke, Gifford, Patalidis & Dumont [57] ABSTRACTA fluid system having a variable displacement pump fluidly connectedthrough valving means to a fluid linear actuator in such a manner thatfluid pressure directed to one portion of the linear actuator causesexpansion thereof at a selected rate, while fluid pressure directed toanother portion of the linear actuator causes contraction thereof at thesame rate. The fluid linear actuator comprises a main fluid cylinderhaving a piston reciprocably mounted therein, with a piston rodextending from one side of the piston and externally of the main fluidcylinder. The piston divides the main fluid cylinder into expansiblepressure chambers, one of which is exposed to the rod carrying side ofthe piston to generate a force thereagainst when pressurized to move thepiston in one direction, while the opposite side of the piston isexposed to the pressure of thefluid in the other pressure chamber whichexerts a force to move the piston in an opposite direction. A secondaryfluid cylinder is carried by the main fluid cylinder with thelongitudinal axes of the secondary and main fluid cylinders beingradially spaced and parallel to one another. The secondary cylinder hasa piston reciprocably mounted therein forming one pressure chamber onthe side of the piston in which a sec- 0nd piston rod is mounted andwhich piston rod extends from the secondary cylinder for connection withthe piston rod of the main fluid cylinder, such that the main cylinderpiston and the secondary piston reciprocate together as a unit. Theeffective pressure responsive area of the side of the piston in the maincylinder opposite the rod carrying side thereof is substantially equalto the sum of the effective pressure responsive area of the rod side ofthe piston in the main cylinder and the effective pressure responsivearea of the rod side of the piston within the secondary cylinder, suchthat the force generated against the two pistons for moving the pistonsin one direction is equal to the force exerted against the pistons formoving the same in the opposite direction. Valving means are providedfor interconnecting the pressure chambers associated with both the mainand secondary fluid cylinders so that the rate of expansion andcontraction of the cylinders may be selectively increased as the forceexerted on the pistons is decreased and vice versa to provide a fluidlinear actuator having a wide speed-force range.

10 Claims, 4 Drawing Figures FLUID SYSTEM BACKGROUND OF THE INVENTION 1.Field of the Invention The present invention relates to fluid systemsand, in particular, to a fluid system employing a fluid linear actuatorso constructed that the forces generated for expanding and contractingthe actuator are equal.

2. Description of the Prior Art In fluid systems utilizing a variabledisplacement pump and a fluid cylinder for operation over a wide rangeof speeds, the variable force from the fluid cylinder is always constantand at a value that is determined by a relief valve setting and the areaof the piston within the fluid cylinder. Since horsepower is a functionof speed, force, and a constant, the horsepower characteristics of sucha system will increase with an increasing speed, whereas, in mostapplications the load horsepower requirements generally remain constanteven with increases and decreases in speed. Since it is a conventionalpractice to base the design of a system upon the horsepower needed todrive the system pump at maximum speed and pressure ratings rather thanthe load power requirements, the system may be overdesigned as the loadpower requirements may never approach the value of the maximum pumpspeed and pressure rating. For example, in a milling machine the forcerequirement decreases as the cutting speed increases for a givenmaterial, and thus, the load horsepower requirements for such anapplication may be almost constant. This characteristic can of course beeasily matched by a constant displacement pump and a variabledisplacement rotary motor, however, since the linear operation of afluid cylinder is required in many applications and there is no suchthing as a continuously variable area cylinder, the system must beoverdesigned to provide the necessary characteristics. If a continuouslyvariable area cylinder were available, the system employing such acylinder would require a pump of considerably less output than a fixedarea cylinder for the same speed range.

It would therefore be desirable to provide a linear actuator in whichthe rate of expansion and/or contraction thereof may be varied with acorresponding increase or decrease in the force characteristics of theactuator, and which actuator has speed-force characteristics approachingthe heretofore unavailable continuously variable area fluid cylinder.

SUMMARY OF THE PRESENT INVENTION The present invention, which will bedescribed subsequently in greater detail, comprises a linear actuatorwith a main fluid cylinder having a main piston reciprocably mountedtherein dividing the main cylinder into two pressure chambers. The mainpiston carries a rod extending through one of the pressure chambers andexternally of the fluid cylinder to reciprocally drive an external load.A secondary cylinder, fixedly attached to the main cylinder, has asecondary piston reciprocably mounted therein and forming a pressurechamber. A piston rod, extending from the secondary piston through thelast mentioned pressure chamber, is operably coupled to the maincylinder piston rod, such that the main and secondary pistons move as aunit.

The effective pressure responsive area of the side of the main pistonopposite the rod side thereof is equal to the sum of the effectivepressure responsive area of the rod side of the main piston and theeffective pressure responsive area of the rod side of the secondarypiston, such that the force for expanding the linear ac tuator is equalto the force for contracting the linear actuator. The secondary and maincylinders are connected to one another such that their longitudinal axesare radially spaced from and are parallel to one another. Suitablevalving means are provided for interconnecting the pressure chambers ofthe main and secondary cylinders so as to provide, in effect, a variablearea linear actuator.

It is therefore an object of the present invention to provide a fluidsystem having a new and improved linear actuator which has a widespeed-force range.

Other objects, advantages, and applications of the present inventionwill become apparent to those skilled in the art of fluid systems andlinear actuators when the accompanying description of some examples ofthe best modes contemplated for practicing the invention is read inconjunction with the accompanying drawing.

BRIEF DESCRIPTION OF Til-IE DRAWING The description herein makesreference to the accompanying drawing wherein like reference numeralsrefer to like parts throughout the several views, and in which:

FIG. 1 is a schematic circuit diagram of a fluid system illustrating apreferred embodiment of the present invention;

FIG. 2 is a schematic circuit diagram illustrating another example ofthe present invention;

FIG. 3 is a schematic circuit diagram illustrating yet another exampleof the present invention; and

FIG. 4 is a fragmentary view of the circuit illustrated in FIG. 3incorporating a modification of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingand, in particular, to FIG. ll, there is illustrated a schematic circuitdiagram of a fluid system Ill) incorporating the principles of thepresent invention and comprising a reversible, variable displacementpump 12 which may be preferably of the axial piston type having portconnections 14 and 16, either of which may be an inlet port or an outletport depending upon the direction of flow from the reversible, variabledisplacement pump 12. The port connections 14 and 16 of the pump 12 are,respectively, fluidly connected to a linear actuator 18 by main conduits20 and 22.

The linear actuator 18 comprises: a main fluid cylinder 24 formed froman enclosed cylindrically shaped tubular member 26, the interior ofwhich forms a longitudinal bore 28 within which is reciprocably mounteda piston 30. Piston 30 divides the bore 28 into two expansible pressurechambers 32 and 34. The side 36 of the piston 30 exposed to the pressureof the fluid within the expansible pressure chamber 34 has a piston rod38 which extends therefrom and externally of the main fluid cylinder 24through an aperture 40 in one of the closed ends of the cylinder 24.When the pressure chamber 34 is communicated to a source of fluidpressure, such as the pump 12, the pressure of the fluid within thechamber 34 exerts a force against the annular surface of piston side 36,which moves the piston 30 leftwardly as viewed in FIG. 1, so as tocontract the main fluid cylinder 24. When the pressure chamber 32 iscommunicated to the high pressure side of pump 12, the pressure of thefluid within the chamber 32 exerts a force against the piston side 42 tomove the piston 30 to the right as viewed in FIG. ll, so as to expandthe main fluid cylinder 24.

The linear actuator 18 further comprises a secondary fluid cylinder 44formed from an enclosed tubular member 46 having an internal bore 48,within which is reciprocably mounted a piston 50 which, in turn, dividesthe interior bore 48 into two expansible chambers 52 and 54. One side 56of the piston 50 has a piston rod 58 extending therefrom and externallyof the fluid cylinder 44 through an aperture 60 in one of the closedends of the member 46. The external ends of the piston rods 38 and 58are coupled to one another by a connecting member 62, such that bothpistons 30 and 50 reciprocate together as a unit. The side 56 of thepiston 50 is adapted to be communicated to fluid pressure to exert aforce on the piston 50 so as to move the same leftwardly as viewed inFIG. 1, while the chamber 52 on the opposite side of the piston 50 isnormally vented to the atmosphere through vent aperture 64 or optionallythe chamber 52 may be connected to a reservoir (not shown).

The pump port connection 14 is connected to the pressure chamber 32 ofthe main fluid cylinder 24 by the conduit 20. The port connection 14 mayalso be connected to the pressure chamber 34 via conduits 66 and 22 whenon-off valves 70 and 72 are opened. A branch conduit 74, connected tothe conduit 66 intermediate the on-off valves 70 and 72, permits fluidcommunication between the pump port connection 14 and the secondarycylinder chamber 54 when the on-off valve 70 and an on-off valve 76 inconduit 74 are both opened.

Conduit 22 communicates the port connection 16 of the pump 12 to themain cylinder pressure chamber 34 through another on-off valve 78,'whilea branch con duit 80 communicates the secondary cylinder chamber 54 tothe port connection 16 via an on-off valve 82. As will be explained ingreater detail hereinafter, the pump port connections 14 and 16 may befluidly connected to any of the pressure chambers 32, 34 and/or 54 byproper manipulation of the on-off valves 70, 72, 76, 78 and 82.

A shuttle relief valve 84 of a conventional construction is providedbetween conduits 20 and 22 so that irrespective of which pump portconnection 14 or 16 high pressure fluid is being delivered from, anyexcessive fluid in the system will be exhausted back to the low pressureside of the pump 12 so as to prevent over pressurization of the system10.

The effective pressure responsive area of the piston side 42 exposed tothe fluid pressure communicated to the main pressure chamber 32 is equalto the sum of the pressure responsive area of piston side 36 exposed tothe pressure of the fluid within the main pressure chamber 34 and theeffective pressure responsive area of the piston side 56 exposed to thefluid in the secondary pressure chamber 54, such that when fluid underpressure is communicated to the pressure chamber 32 a force of apredetermined magnitude will be exerted against the piston 30 to shiftthe piston rightwardly as viewed in FIG. 1 and expand the linearactuator 18. When fluid at the same pressure is communicated to bothpressure chambers 34 and 54, a combined force is generated against thepistons 30 and 50, which force tends to move the pistons leftwardly andcontract the linear actuator 18. That is, the force exerted on thesurface 42 of the piston 30 to expand the linear actuator 18 issubstantially equal to the sum of the forces exerted against the pistonsides 36 and 56 to contract the linear actuator 18.

By properly controlling the amount of fluid communicated from the pump12 to the pressure chambers 32, 34 and 54, the rate of movement of thepistons 30 and 50 rightwardly or leftwardly, that is the expansion andcontraction of the pistons in the fluid cylinders 24 and 44 as well asthe net force acting on the pistons 30 and 50, can be varied over a widespeed-force range.

For example, when the on-off valves and 82 are both closed and the onoffvalves 72, 76 and 78 are opened, fluid pressure is delivered from thepump 12 through the port connection 14 to the pressure chamber 32 of themain fluid cylinder 24, while the pressure chambers 34 and 54 areexhausted to the port connection 16, thereby forming a closed loopcircuit, that is the amount of fluid delivered from pump 12 to chamber32 equals the amount of fluid exhausted from chambers 34 and 54 back topump 12. It can thus be seen that during expansion of the linearactuator 18 the full pressure responsive area of the piston side 30 isexposed to fluid pressure, generating a maximum force and requiring amaximum flow of fluid to move the same, and thus the linear actuator 18will expand at a relatively low rate while generating a maximum outputforce. When the variable displacement pump is reversed so that highpressure fluid is delivered from the port connection 16 to the pressurechambers 34 and 54, a maximum force is exerted on the pistons 30 and 50to cause contraction of the linear actuator 18.

When, for example, on-off valves 72 and 82 are both closed while on-offvalves 70, 76 and 78 are opened, fluid pressure delivered from theconnection port 14 of pump 12 will be communicated to both pressurechambers 32 and 54, while the chamber 34 is exhausted back to the pump12 via port connection 16. It can thus be seen that the net forcedifferential acting on the two pistons 30 and 50 to cause expansion ofthe device will be equal to the difference between the effective area ofthe main piston side 42 and the area of the secondary piston side 56.Since the area of piston side 56 is equal to the cross-sectional area ofthe main piston rod 38, the net effective responsive area tending toexpand the linear actuator 18 will be equal to the area of the pistonside 42 minus the area of the main piston rod 38. Thus, an intermediateforce for expanding the actuator 18 may be obtained. Since chambers 32and 54 are in communication and chamber 54 is being contracted aschamber 32 is expanded, the fluid in chamber 54 will flow into chamber32, which results in a faster rate of expansion of the actuator 18. Whenthe flow from the pump 12 is reversed, fluid pressure will becommunicated to chamber 34 to contract the actuator 18 at the same rateand at the same force as during expansion, while the pressure chamber 32is exhausted to the pump 12 and chamber 54. It should be noted that asthe pistons 30 and 50 are being stroked to expand the actuator 18, thefluid within the pressure chamber 54 is being compressed and cycled intochamber 32, and when the actuator 18 is being contracted and thepressure chamber 32 is being contracted, a portion of the fluid isreturned to pressure chamber 54 as the same is being expanded, whereby asmaller volume of fluid is required from the pump 12, thereby increasingboth the speed of contraction and expansion of the actuator, while theoutput force is decreased.

When the on-off valves 76 and 78 are closed and the on-off valves 70, 72and 82 are opened, fluid underpressure will be delivered from portconnection 14 to both chambers 32 and 34, while pressure chamber 54 isbeing exhausted. Since the net pressure responsive area tending toexpand the actuator 18 will be equal to the cross-sectional area of thepiston rod 38, a relatively small force is obtained for expanding theactuator 18. When the piston 30 is moved rightwardly to expand chamber32, fluid from the pressure chamber 34 is cycled back to the chamber 32,and when the piston 30 is moved leftwardly to contract chamber 32, fluidfrom chamber 32 will be directed to both the chamber 34 and the pump 12,such that the amount of fluid being directed to and from the pump 12 isequal during both the expansion and the contraction phases of theactuator. Since a small volume of fluid is required from the pump 12,the actuator 18 will expand and contract at a substantially greatervelocity than in the heretofore described examples, while the forceimparted by the actuator will be reduced to a minimum.

Referring now to FIG. 2 there is illustrated another example of thepresent invention in the form of a fluid system 89 comprising a fluidlinear actuator 90 having the same components as the actuator 18 andwhich components are designated by the same reference numerals as theactuator 18 illustrated in FlG. 1. The actuator 90 is illustrated ascomprising the main cylinder 24 and two secondary cylinders 44 and 44',the piston rods 58 and 58' of which are coupled to the main piston rod38 by connecting member 62, such that the pistons 50 and 50' of the twosecondary cylinders move with main piston 30 as a unit. The effectivepressure responsive area of the piston side 42 is substantially equal tothe sum of the pressure responsive areas of the main piston side 36 andthe piston sides 56 and 56 of the two secondary pistons 50 and 50,respectively. In the embodiment illustrated in FIG. 2, the pressurechamber 34 is in constant fluid communication with the two secondarypressure chambers 54 and 54' via apertures 92 and 93 extending throughthe walls of the tubular members 26, 46' and 46.

The port connections 14 and 16 of the pump 12 are respectively connectedby conduits 93 and 95 to the port connections of a directional controlvalve 94 which is adapted to selectively communicate fluid from the portconnections 14 and 16 to conduits 96 and 98 which, in turn, arerespectively communicated to the main fluid chamber 32 and the fluidchambers 34, 54 and 54' of the actuator 90. In the embodimentillustrated, high pressure fluid is delivered from the unidirectionalpump 12 through port connection 16.

Movement of the directional control valve 94 is controlled by a pilotvalve 100 having an inlet port in constant communication with the highpressure delivery port connection 16 of the pump 12. When the pilotvalve 100 is shifted to one position, high pressure fluid from theconduit 95 is communicated via a line 101 to one side of the directionalcontrol valve 94 to shift the same, whereby fluid pressure iscommunicated to conduit 96, while the other conduit 98 is communicatedto the pump port connection 14. When the pilot valve 100 is shifted inthe opposite direction, fluid pressure is communicated via line 103 tothe opposite side of the directional control valve 94 to shift the samein an opposite direction, thereby reversing the flow of fluid betweenthe pump 12 and the actuator 90, that is, fluid pressure from pump portconnection 16 is communicated to the conduit 98 while fluid is returnedto pump port connection from the conduit 96. A shuttle valve 97, similarto the hereinbefore described shuttle valve 84, is provided in thesystem 89 between conduits 96 and 98 and functions to prevent overpressurization of the actuator 90, and at the same time relieves anyexcess fluid in the event the aforementioned area differentials are notequal due to variations in manufacturing tolerances.

The pilot valve and the directional control valve 94 are conventional intheir construction and operation and thus a further detailed descriptionof their construction and operation is not deemed to be necessary.

The conduits 96 and 98, respectively, have pilot operated check valves102 and 104 and the conduit 96 is provided with a sequence valveintermediate the check valve 102 and the chamber 32. The check valve 102and the sequence valve 105 permit unrestricted flow from the pump 12through directional control valve 94 and conduit 96 to the pressurechamber 32, wherein the pressure exerts a force on the piston 30 to movethe same rightwardly (as viewed in H6. 2) to expand the fluid cylinders,while the check valve 104 permits unrestricted flow from the pump 12through directional control valve 94 and conduit 98 to the pressurechambers 34, 54 and 54', wherein the pressure exerts a force on thepiston 30, 50 and 50 to move them leftwardly (as viewed in FIG. 2) toretract the pistons in the fluid cylinders. However, the check valvesnormally function to prevent the flow of fluid through conduits 96 and98 from the pressure chambers back to the pump 12. In order to permit abackflow from the actuator to the pump 12 through-either of the checkvalves 102 or 104, the pilot portions thereof are respectivelycommunicated via lines 106 and 108 to the high pressure outlet lines 103and 101 of the pilot valve 100, such that when the valve 100 is shiftedin a manner to communicate pressure via line 101 to one side of thedirectional control valve 94 to actuate the same so as to communicatehigh pressure fluid from the pump 12 to the conduit 96, the pilot checkvalve 104 is opened by the pressure fluid from line 101 and permits thefluid in conduit 98 to flow from the pressure chambers 34, 54 and 54back to the pump port connection 14; whereas when the pilot valve 106 isshifted in the opposite direction so as to communicate pressure fluidvia line 103 to the directional control valve 94 to actuate the same soas to communicate-pressure fluid from pump 12 to chambers 34, 54 and 54via conduit 98, the check valve 102 is opened by pressure from line 103and permits the fluid in conduit 96 to flow from pressure chamber 32back to pump 12. When the system 89 is not being operated but theactuator 90 is required to hold a load in a fixed, for example avertical or raised position, the check valves 102 and 104 will remainclosed and thus function to provide a positive hold of the position ofthe actuator 90 since the fluid within the chambers 32, 34, 54 and 54'is trapped therewithin and the pistons are restrained from reciprocalmovement, whereby the position of the load held by the actuator 90 isfixed.

Referring now to FIG. 3 wherein there is illustrated a schematic circuitdiagram of a fluid system 110 embodying a third example of the presentinvention and comprising a linear actuator 112, which is the same as thelinear actuator 18 disclosed in FIG. 1, and the corresponding elementsof both actuators 18 and 112 are identified by the same numeraldesignations. The pump 12 is of the reversible, variable displacementtype having the outlet connection ports 14 and 16 respectively in fluidcommunication with the pressure chambers 32 and 34 of the main cylinder24 via conduits 114 and 116. Conduit 114 also communicates with avariable displacement motor 118 via a branch conduit 120, while theconduit 116 communicates with a second variable displacement motor 122via a second branch conduit 124. The outlet ports of the variabledisplacement motors 118 and 122 communicate with each other and to thepressure chamber 54 of the secondary cylinder 44 via conduit 126.

The variable displacement motors 118 and 122 are coupled to each otherby a suitable mechanical coupling means 128 in such a manner that as thedisplacement of the variable displacement motor 118 is increased thedisplacement of the variable displacement motor 122 is decreased, andvice versa.

An example of constructing the variable displacement motors 118 and 122to achieve a synchronized operation is illustrated in FIG. 4 ascomprising a unitary housing construction 140 having a longitudinal bore142 enclosed at its opposite ends by valve plates 144 and 146 thatdefine the inlets and outlets of the motors 118 and 122, respectively,and which communicate in the conventional manner with a plurality ofarcuately spaced, parallel cylinder bores 148 and 149 respectivelyformed in a pair of axially spaced cylinder barrels 150 and 152respectively. The cylinder barrels 150 and 152 are rotatably mounted ona common shaft 154 which, in turn, is rotatably supported at itsopposite ends by bearings 156 and 158 carried in the valve plates 144and 146, respectively. The cylinder barrel 150 has a plurality ofpistons 160 reciprocably mounted within each of the cylinder barrelbores 148, with the outer ends of each of the pistons slidably engaginga thrust bearing surface 162 formed on one side of a triangularly shapedthrust block 163 which is adapted to rotate about an axis 164 that istransversely disposed with respect to the axis of rotation of thecylinder barrels 150 and 152. Similarly the cylinder barrel 152 has aplurality of pistons 166 reciprocably mounted within each of thecylinder barrel bores 149 with each piston 166 having outer ends thatslidably engage a thrust bearing surface 168 formed on another of thetriangular sides of the thrust block 163. As can be seen with referenceto FIG. 3, as the thrust block 163 is rotated about the axis 164 toincline the thrust bearing surface 168 with respect to the axis ofrotation of the cylinder barrel 152, the displacement of the motor 122is increased, while at the same time the thrust bearing surface 162 isrotated toward a position wherein it lies in a plane that isperpendicular to the longitudinal axis of the cylinder barrel 150 andthus decreases the stroke of the pistons 160 therewithin and thusdecreases the displacement of the motor 118. Similarly when the thrustblock 163 is rotated in an opposite direction the displacement of themotor 118 is increased while the displacement of the motor 122 isdecreased. If the swash block is made symmetrical, the displacement ofone of the motors will be decreased in proportion to the increase of thedisplacement of the other motor and thus the synchronized manner ofoperation hereinbefore described can be easily achieved.

In operation, the displacements of the variable displacement motors 1 18and 122 are preset to allow fluid to flow through at a predeterminedrate. If, for example, the volume of the pressure chamber 32 is adaptedto receive 14 cubic inches of fluid, while the volume of the chamber 34is adapted to receive 4 cubic inches, the effective rate of expansionand contraction of the chambers may be selectively varied over a widerange in the following manner. For example, when the pump 12 isdelivering 10 cubic inches per second and the variable displacementmotor 118 is permitting 4 cubic inches of flow therethrough as thechamber 54 is compressed, that is, chamber 32 is expanding, a total of14 cubic inches per second would be delivered to the chamber 32 and thepistons would advance 1 inch. However, if the variable displacementmotor 118 is preset to permit only 1 cubic inch of flow between thechambers 54 and 32, the pistons will advance only a distance of 0.786inch. In this condition, the variable displacement motor 122 mustdeliver 2.14 cubic inches, which in combination with the 7.86 cubicinches of fluid exhausted from chamber 34 provides a total of 10 cubicinches per second of fluid to the pump and thus the amount of fluiddelivered to the pump 12 is equal to the output.

It can be seen that by decreasing the displacement of the motor 118 tovalues less than 4 cubic inches, while increasing the displacement ofthe variable displacement motor 122 up from a zero displacement to atotal of 4 cubic inches, the rate of expansion or contraction, dependingupon the direction of flow of pressure fluid from pump 12, may beselectively varied over the entire flow range of the actuator 112.

It can thus be seen that the present invention has provided a fluidsystem which utilizes a main fluid cylinder in conjunction with one ormore secondary fluid cylinders and in which the effective pressureresponsive area for expansion of the cylinders is equal to the effectivepressure responsive areas for contraction of the cylinders and in whichthe rate of flow of fluid and force exerted by the fluid cylinders maybe varied over a wide range of operation.

Although several embodiments of the present invention have beendisclosed, it is to be understood by those skilled in the art of fluidsystems that other embodiments may be had and modifications may be madeto the present systems without departing from the spirit of theinvention or from the scope of the appended claims.

What is claimed is as follows:

1. A fluid system comprising:

a first tubular member having an internal bore closed at its oppositeends;

a piston reciprocally mounted within said bore and dividing said boreinto two expansible pressure chambers;

a piston rod carried on one side of said piston and extending externallyof said tubular member through one of said closed ends;

first valve means for communicating fluid pressure to the pressurechamber associated with the rod side of said piston to generate a forceagainst said piston to move same in one direction;

second valve means for communicating fluid pressure to the pressurechamber on the side of said piston opposite said rod to generate a forceagainst said piston to move said piston in an opposite direction;

a second tubular member having an internal bore closed at one end, saidsecond tubular member having a longitudinal axis radially spaced fromandparallel to the longitudinal axis of said first tubular member;

a second piston reciprocally mounted within said second tubular memberbore and defining an expansible pressure chamber;

a piston rod carried on one side of said second piston and extendingthrough the pressure chamber of said second tubular member andexternally of said second tubular member through said closed end;

means for connecting the extended external ends of said piston rods suchthat said pistons reciprocate together;

third valve means for communicating a pressure fluid to the pressurechamber of said second tubular member for exerting a force on the rodside of said second piston for moving said second piston in onedirection;

the effective pressure responsive areas of the rod sides of said firstmentioned piston opposite said rod being substantially equal to the sumof the effective pressure responsive areas of the rod sides of saidfirst mentioned piston and said second piston;

a fluid pump having an outlet for delivering fluid under pressure at aselected rate of flow and an inlet for receiving fluid at the sameselected rate of flow;

said first and second valve means being adapted to, first, communicatesaid pump outlet to the rod side pressure chambers of said first tubularmember and said second tubular member, while communicating said pumpinlet to the pressure chamber of said first tubular member opposite saidrod for moving said first and second pistons in said one direction at apredetermined velocity and with a predetermined force;

said first and second valve means being adapted to,

second, communicate said pump outlet to the pressure chamber of saidfirst tubular member opposite said rod and to communicate said pumpinlet to the rod side pressure chambers of said first tubular member andsaid second tubular member for moving said first and second pistons insaid opposite direction at said predetermined velocity and with a saidpredetermined force.

2. A fluid system comprising:

a first tubular member having an internal bore closed at its oppositeends;

a piston reciprocally mounted within said bore and dividing said boreinto two expansible pressure chambers;

a piston rod carried on one side of said piston and extending externallyof said tubular member through one of said closed ends;

first valve means for communicating fluid pressure to the pressurechamber associated with the rod side of said piston to generate a forceagainst said piston to move same in one direction;

second valve means for communicating fluid pressure to the pressurechamber on the side of said piston opposite said rod to generate a forceagainst said piston to move said piston in one direction;

second valve means for communicating fluid pressure to the pressurechamber on the side of said piston opposite said rod to generate a forceagainst said piston to move said piston in an opposite direction;

a second tubular member having an internal bore closed at one end, saidsecond tubular member having a longitudinal axis radially spaced fromand parallel to the longitudinal axis of said first tubular member;

a second piston reciprocally mounted within said second tubular memberbore and defining an expansible pressure chamber;

a piston rod carried on one side of said second piston and extendingthrough the pressure chamber of said second tubular member andexternally of said second tubular member through said closed end;

means for connecting the extended external ends of said piston rods suchthat said. pistons reciprocate together;

third valve means for communicating a pressure fluid to the pressurechamber of said second tubular member for exerting a force on the rodside of said second piston for moving said second piston in onedirection;

the effective pressure responsive areas of the rod sides of said firstmentioned piston opposite said rod being substantially equal to the sumof the effective pressure responsive areas of the rod sides of saidfirst mentioned piston and. said second piston;

the rod side pressure chamber of said first tubular member being inconstant fluid communication with said second tubular member pressurechamher;

.a first conduit means connecting said second valve means to thepressure chamber of said first tubular member opposite said rod;

second conduit means connecting the rod side pressure chamber of saidfirst tubular member and the pressure chamber of said second tubularmember to said first valve means;

said first and second valve means being adapted to selectively directfluid under pressure to one of said conduit means while exhausting fluidpressure from the other of said conduit means;

check valve means disposed in each of said conduit means for normallypreventing flow from said pressure chambers to said first and secondvalve means, said check valve means being responsive to open and permitfluid to flow from said pressure chambers through their associatedconduits when said first and second valve means are actuated to directfluid pressure to the other of said conduits. r 3. The fluid systemdefined in claim 1 wherein said first and second valve means are furtheradapted to communicate said pump outlet to the rod side pressurechambers of said first tubular member and said second tubular memberwhile the pressure chamber opposite said rod of said first tubularmember is communicated to said pump inlet to exert a second force onsaid piston the outlet of said fluid pump to said pressure chamber ofsaid first tubular member opposite said rod, while communicating theinlet of said pump to the rod side pressure chambers of said firsttubular member and said second tubular member to move said piston insaid other direction at the said second velocity and with said secondforce.

4. The fluid system defined in claim 1 wherein said first and secondvalve means are further adapted to communicate said pump outlet to bothfluid pressure chambers of said first tubular member and communicatesaid inlet of said pump to said second tubular member pressure chamberto move said piston in said one direction at a second velocity greaterthan said pre determined velocity and a second force less than saidpredetermined force, and said first and second valve means being adaptedto communicate the outlet of said pump to said second tubular memberpressure chamber while communicating the inlet of said pump to bothpressure chambers of said first tubular member to move said pistons insaid other direction at said second speed and second force.

5. A fluid system comprising:

a first tubular member having an internal bore closed at its oppositeends;

a piston reciprocally mounted within said bore and dividing said boreinto two expansible pressure chambers;

a piston rod carried on one side of said piston and extending externallyof said tubular member through one of said closed ends;

first valve means for communicating fluid pressure to the pressurechamber associated with the rod side of said piston to generate a forceagainst said piston to move same in one direction;

second valve means for communicating fluid pressure to the pressurechamber on the side of said piston opposite said rod to generate a forceagainst said piston to move said piston in one direction;

second valve means for communicating fluid pressure to the pressurechamber on the side of said piston opposite said rod to generate a forceagainst said piston to move said piston in an opposite direction;

a second tubular member having an internal bore closed at oneend, saidsecond tubular member having a longitudinal axis radially spaced fromand parallel to the longitudinal axis of said first tubular member;

a second piston reciprocally mounted within said second tubular memberbore and defining an expansible pressure chamber;

a piston rod carried on one side of said second piston and extendingthrough the pressure chamber of said second tubular member andexternally of said second tubular member through said closed end;

means for connecting the extended external ends of said piston rods suchthat said pistons reciprocate together;

third valve means for communicating a pressure fluid to the pressurechamber of said second tubular member for exerting a force on the rodside of said second piston for moving said second piston in onedirection;

the effective pressure responsive areas of the rod sides of said firstmentioned piston opposite said rod being substantially equal to the sumof the cffective pressure responsive areas of the rod sides of saidfirst mentioned piston and said second piston;

a plurality of said second tubular members, each having a second pistonconnected to said first piston, the effective pressure responsive areaof the side of said rod being substantially equal to the sum of theeffective pressure responsive area of the rod side of said first pistonand the effective pressure responsive areas of the sides of said secondpistons associated with said second tubular members.

6. The system defined in claim 2 wherein said check valves are pilotoperated valves and are operable to open in response to a controlpressure communicated to said first and second valve means.

7. A linear actuator comprising:

a first tubular member having an internal bore enclosed at its oppositeends;

a piston reciprocally mounted within said bore and dividing said boreinto two expansible pressure chambers;

a piston rod carried on one side of said piston and extending throughone of said pressure chambers and externally of said tubular member toone of said closed ends;

said one pressure chamber being adapted to be communicated to a sourceof fluid pressure to generate a force against the rod side of saidpiston to move same in one direction;

the pressure chamber opposite said rod being adapted to be communicatedto said source of fluid pressure to generate a force against the otherside of said piston to move same in an opposite direction;

a second tubular member having an internal bore at one end, said secondtubular member having a longitudinal axis radially spaced from andparallel to the longitudinal axis of said first tubular member;

a second piston reciprocably mounted within said second tubular memberbore and defining an expansible pressure chamber therein;

a piston rod carried on one side of said second piston and extendingthrough said last mentioned pressure chamber and externally of saidsecond tubular member through said closed end;

means for connecting the extended external ends of said piston rods suchthat said first and second pistons reciprocate together;

said second tubular member pressure chamber being adapted to becommunicated to a source of pressure fluid to exert a force on the sideof said second piston carrying said piston rod to move said secondpiston in said one direction; and

the effective pressure responsive area of the side of said first pistonopposite said rod being substantially equal to the sum of the effectivepressure responsive areas of the rod side of said first piston and theside of said second piston associated with said second tubular memberpressure chamber,

said linear actuator further comprising a plurality of said secondtubular members, each having a second piston connected to said firstpiston, the effective pressure responsive area of the side of said firstpiston opposite said rod being substantially equal to the sum of theeffective pressure responsive areas of the rod side of said first pistonand of the rod sides of said second pistons.

8. A linear actuator comprising:

a first tubular member having an internal bore closed at its oppositeends;

a piston reciprocally mounted within said bore and dividing said boreinto two expansible chambers; a piston rod carried on one side of saidpiston and extending externally of said tubular member through one ofsaid closed ends;

a second tubular member having an'internal bore closed at one end, saidsecond tubular member having a longitudinal axis radially spaced fromand parallel to the longitudinal axis of said first tubular member,

a second piston reciprocally mounted within said second tubular memberbore and defining an expansible pressure chamber;

a piston rod carried on one side of said second piston and extendingthrough said second tubular member pressure chamber and externally ofsaid second tubular member through said closed end;

means for connecting the extended external ends of said piston rods suchthat said pistons reciprocate together;

means for variably supplying fluid pressure to said chambers to therebyvary the force generated by said linear actuator said means comprising afirst and second variable displacement motoring means connected withsaid chambers and control means coupling said first and second variabledisplacement means such that the displacement of one of said motoringmeans is increased as the other of saidmotoring means is decreased.

9. A fluid system comprising:

a first tubular member having an internal bore enclosed at its oppositeends;

a source of fluid pressure;

a piston reciprocally mounted within said bore and dividing said boreinto two expansible pressure chambers;

a piston rod carried on one side of said piston and extending throughone of said pressure chambers and externally of said tubular member toone of said closed ends;

said pressure chamber on the rod side of said piston being adapted to becommunicated to said source of fluid pressure to generate a forceagainst said piston to move same in one direction;

the pressure chamber on the side of said piston opposite said rod beingadapted to be communicated to said source of fluid pressure to generatea force against said piston to move same in an opposite direction;

a second tubular member having an internal bore at one end, said secondtubular member having a longitudinal axis radially spaced from andparallel to the longitudinal axis of said first tubular member;

a second piston reciprocably mounted within said second tubular memberbore and defining a second expansible pressure chamber therein;

a piston rod carried on one side of said second piston and extendingthrough said second pressure chamber and externally of said secondtubular member through said closed end;

means for connecting the extended external ends of said piston rods suchthat said first and second pislld tons reciprocate together;

first and second variable displacement motoring means, said firstmotoring means having an inlet connected to the pressure chamber on therod side of said first piston and an outlet connected to said secondpressure chamber, said second motoring means having an inlet connectedto pressure chamber opposite said rod and an outlet connected to saidsecond pressure chamber, displacement control means coupling said firstand second motoring means such that the displacement of one of saidmotoring means is increased as the displacement of the other isdecreased;

said second tubular member pressure chamber being adapted to beselectively communicated to said source of pressure fluid through saidfirst and second motoring means to exert a force on the rod side of saidsecond piston to move said second piston in said one direction; and

the effective pressure responsive area of the side of said first pistonopposite said. rod being substantially equal to the sum of the effectivepressure responsive area of the rod side of said first piston and theeffective pressure responsive area of said side of said second pistonassociated with said second tubular member pressure chamber.

10. The fluid system defined in claim 9 wherein said first and secondmotoring means comprise:

a housing having a longitudinally disposed bore;

a pair of axially spaced cylinder barrels rotatably mounted about acommon axis in said housing bore, each of said cylinder barrels having aplurality of arcuately spaced, parallel bores opening to the oppositefaces of each of said cylinder barrels;

first valving means at one end of said housing bore communicating oneface of one of said cylinder barrels with high and low pressure passagesdefining said first motoring means inlet and outlet;

second valving means at the other end of said housing bore communicatingone face of the other of said cylinder barrels with high and lowpressure passages defining said second motoring means inlet and outlet;

first piston means reciprocably mounted in said plurality of bores insaid one cylinder barrel and having outer ends extending therefrom forengagement with a thrust surface, the amount of inclination of whichcontrols the amount of displacement of said one cylinder barrel and thusthe amount of displacement of said first motoring means;

a second piston means reciprocably mounted in said plurality of bores insaid other cylinder barrel and having outer ends extending therefrom forengagement with a second thrust surface, the amount of inclination ofwhich controls the displacement of said other cylinder barrel and] thusthe displacement of said second motoring means, said first and secondthrust surfaces being formed on the opposite side of a common swashplate adapted to be pivoted in one direction to increase thedisplacement of one of said motoring means while simultaneously and insynchronism decreasing the dis placement of the other motoring means.

:I: :9: 1|: m at;

Patent No. 3 I 792 l 43 February 19, 1974 Dated Inventor(s) (SEAL)Atteat:

EDWARD .FLETCHER, JR. Attesting Dfficer FORM PO-IOSO (10-69) Robert E.Soheafer It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Col. 3, line 60, delete "main".

Col. 4, line l3, after "pistons", insert -30 and 5 O-.

Col. 5, line 49, delete "93" and insert 91--. N

Col. 6, line 34, after "pistons", insert --30, 50 and 50--;

line 35, after "valves", insert l02 and l04-; line 52, delete "106", andinsert lOO-.

Col. 7, line 43, after "pistons", insert -l60-.

Col. 9, delete lines 66-67.

Col. 10, delete lines 1-2. 7

Col. 11, delete lines 36-39.

Signed and sealed this 11th day of June 197A.

0. 'MARSHALL DANN Commissioner of Patents USCOMlM-DC 603 76'P69 u.s.GOVERNMENT PRINTlNG OFFICE: um c ase-3:5

1. A fluid system comprising: a first tubular member having an internalbore closed at its opposite ends; a piston reciprocally mounted withinsaid bore and dividing said bore into two expansible pressure chambers;a piston rod carried on one side of said piston and extending externallyof said tubular member through one of said closed ends; first valvemeans for communicating fluid pressure to the pressure chamberassociated with the rod side of said piston to generate a force againstsaid piston to move same in one direction; second valve means forcommunicating fluid pressure to the pressure chamber on the side of saidpiston opposite said rod to generate a force against said piston to movesaid piston in an opposite direction; a second tubular member having aninternal bore closed at one end, said second tubular member having alongitudinal axis radially spaced from and parallel to the longitudinalaxis of said first tubular member; a second piston reciprocally mountedwithin said second tubular member bore and defining an expansiblepressure chamber; a piston rod carried on one side of said second pistonand extending through the pressure chamber of said second tubular memberand externally of said second tubular member through said closed end;means for connecting the extended external ends of said piston rods suchthat said pistons reciprocate together; third valve means forcommunicating a pressure fluid to the pressure chamber of said secondtubular member for exerting a force on the rod side of said secondpiston for moving said second piston in one direction; the effectivepressure responsive areas of the rod sides of said first mentionedpiston opposite said rod being substantially equal to the sum of theeffective pressure responsive areas of the rod sides of said firstmentioned piston and said second piston; a fluid pump having an outletfor delivering fluid under pressure at a selected rate of flow and aninlet for receiving fluid at the same selected rate of flow; said firstand second valve means being adapted to, first, communicate said pumpoutlet to the rod side pressure chambers of said first tubular memberand said second tubular member, while commUnicating said pump inlet tothe pressure chamber of said first tubular member opposite said rod formoving said first and second pistons in said one direction at apredetermined velocity and with a predetermined force; said first andsecond valve means being adapted to, second, communicate said pumpoutlet to the pressure chamber of said first tubular member oppositesaid rod and to communicate said pump inlet to the rod side pressurechambers of said first tubular member and said second tubular member formoving said first and second pistons in said opposite direction at saidpredetermined velocity and with a said predetermined force.
 2. A fluidsystem comprising: a first tubular member having an internal bore closedat its opposite ends; a piston reciprocally mounted within said bore anddividing said bore into two expansible pressure chambers; a piston rodcarried on one side of said piston and extending externally of saidtubular member through one of said closed ends; first valve means forcommunicating fluid pressure to the pressure chamber associated with therod side of said piston to generate a force against said piston to movesame in one direction; second valve means for communicating fluidpressure to the pressure chamber on the side of said piston oppositesaid rod to generate a force against said piston to move said piston inan opposite direction; a second tubular member having an internal boreclosed at one end, said second tubular member having a longitudinal axisradially spaced from and parallel to the longitudinal axis of said firsttubular member; a second piston reciprocally mounted within said secondtubular member bore and defining an expansible pressure chamber; apiston rod carried on one side of said second piston and extendingthrough the pressure chamber of said second tubular member andexternally of said second tubular member through said closed end; meansfor connecting the extended external ends of said piston rods such thatsaid pistons reciprocate together; third valve means for communicating apressure fluid to the pressure chamber of said second tubular member forexerting a force on the rod side of said second piston for moving saidsecond piston in one direction; the effective pressure responsive areasof the rod sides of said first mentioned piston opposite said rod beingsubstantially equal to the sum of the effective pressure responsiveareas of the rod sides of said first mentioned piston and said secondpiston; the rod side pressure chamber of said first tubular member beingin constant fluid communication with said second tubular member pressurechamber; a first conduit means connecting said second valve means to thepressure chamber of said first tubular member opposite said rod; secondconduit means connecting the rod side pressure chamber of said firsttubular member and the pressure chamber of said second tubular member tosaid first valve means; said first and second valve means being adaptedto selectively direct fluid under pressure to one of said conduit meanswhile exhausting fluid pressure from the other of said conduit means;check valve means disposed in each of said conduit means for normallypreventing flow from said pressure chambers to said first and secondvalve means, said check valve means being responsive to open and permitfluid to flow from said pressure chambers through their associatedconduits when said first and second valve means are actuated to directfluid pressure to the other of said conduits.
 3. The fluid systemdefined in claim 1 wherein said first and second valve means are furtheradapted to communicate said pump outlet to the rod side pressurechambers of said first tubular member and said second tubular memberwhile the pressure chamber opposite said rod of said first tubularmember is communicated to said pump inlet to exert a second force onsaid piston of a lesser value than said first predetermined value aNd tomove said pistons in said one direction at a second velocity greaterthan said predetermined velocity but with the same volume of fluid fromsaid pump; and said first and second valve means being adapted toconnect the outlet of said fluid pump to said pressure chamber of saidfirst tubular member opposite said rod, while communicating the inlet ofsaid pump to the rod side pressure chambers of said first tubular memberand said second tubular member to move said piston in said otherdirection at the said second velocity and with said second force.
 4. Thefluid system defined in claim 1 wherein said first and second valvemeans are further adapted to communicate said pump outlet to both fluidpressure chambers of said first tubular member and communicate saidinlet of said pump to said second tubular member pressure chamber tomove said piston in said one direction at a second velocity greater thansaid predetermined velocity and a second force less than saidpredetermined force, and said first and second valve means being adaptedto communicate the outlet of said pump to said second tubular memberpressure chamber while communicating the inlet of said pump to bothpressure chambers of said first tubular member to move said pistons insaid other direction at said second speed and second force.
 5. A fluidsystem comprising: a first tubular member having an internal bore closedat its opposite ends; a piston reciprocally mounted within said bore anddividing said bore into two expansible pressure chambers; a piston rodcarried on one side of said piston and extending externally of saidtubular member through one of said closed ends; first valve means forcommunicating fluid pressure to the pressure chamber associated with therod side of said piston to generate a force against said piston to movesame in one direction; second valve means for communicating fluidpressure to the pressure chamber on the side of said piston oppositesaid rod to generate a force against said piston to move said piston inan opposite direction; a second tubular member having an internal boreclosed at one end, said second tubular member having a longitudinal axisradially spaced from and parallel to the longitudinal axis of said firsttubular member; a second piston reciprocally mounted within said secondtubular member bore and defining an expansible pressure chamber; apiston rod carried on one side of said second piston and extendingthrough the pressure chamber of said second tubular member andexternally of said second tubular member through said closed end; meansfor connecting the extended external ends of said piston rods such thatsaid pistons reciprocate together; third valve means for communicating apressure fluid to the pressure chamber of said second tubular member forexerting a force on the rod side of said second piston for moving saidsecond piston in one direction; the effective pressure responsive areasof the rod sides of said first mentioned piston opposite said rod beingsubstantially equal to the sum of the effective pressure responsiveareas of the rod sides of said first mentioned piston and said secondpiston; a plurality of said second tubular members, each having a secondpiston connected to said first piston, the effective pressure responsivearea of the side of said rod being substantially equal to the sum of theeffective pressure responsive area of the rod side of said first pistonand the effective pressure responsive areas of the sides of said secondpistons associated with said second tubular members.
 6. The systemdefined in claim 2 wherein said check valves are pilot operated valvesand are operable to open in response to a control pressure communicatedto said first and second valve means.
 7. A linear actuator comprising: afirst tubular member having an internal bore enclosed at its oppositeends; a piston reciprocally mounted within said bore and dividing saidbore into two expansible pressure chambers; a piston rod carried on oneside of said piston and extending through one of said pressure chambersand externally of said tubular member to one of said closed ends; saidone pressure chamber being adapted to be communicated to a source offluid pressure to generate a force against the rod side of said pistonto move same in one direction; the pressure chamber opposite said rodbeing adapted to be communicated to said source of fluid pressure togenerate a force against the other side of said piston to move same inan opposite direction; a second tubular member having an internal boreat one end, said second tubular member having a longitudinal axisradially spaced from and parallel to the longitudinal axis of said firsttubular member; a second piston reciprocably mounted within said secondtubular member bore and defining an expansible pressure chamber therein;a piston rod carried on one side of said second piston and extendingthrough said last mentioned pressure chamber and externally of saidsecond tubular member through said closed end; means for connecting theextended external ends of said piston rods such that said first andsecond pistons reciprocate together; said second tubular member pressurechamber being adapted to be communicated to a source of pressure fluidto exert a force on the side of said second piston carrying said pistonrod to move said second piston in said one direction; and the effectivepressure responsive area of the side of said first piston opposite saidrod being substantially equal to the sum of the effective pressureresponsive areas of the rod side of said first piston and the side ofsaid second piston associated with said second tubular member pressurechamber, said linear actuator further comprising a plurality of saidsecond tubular members, each having a second piston connected to saidfirst piston, the effective pressure responsive area of the side of saidfirst piston opposite said rod being substantially equal to the sum ofthe effective pressure responsive areas of the rod side of said firstpiston and of the rod sides of said second pistons.
 8. A linear actuatorcomprising: a first tubular member having an internal bore closed at itsopposite ends; a piston reciprocally mounted within said bore anddividing said bore into two expansible chambers; a piston rod carried onone side of said piston and extending externally of said tubular memberthrough one of said closed ends; a second tubular member having aninternal bore closed at one end, said second tubular member having alongitudinal axis radially spaced from and parallel to the longitudinalaxis of said first tubular member, a second piston reciprocally mountedwithin said second tubular member bore and defining an expansiblepressure chamber; a piston rod carried on one side of said second pistonand extending through said second tubular member pressure chamber andexternally of said second tubular member through said closed end; meansfor connecting the extended external ends of said piston rods such thatsaid pistons reciprocate together; means for variably supplying fluidpressure to said chambers to thereby vary the force generated by saidlinear actuator said means comprising a first and second variabledisplacement motoring means connected with said chambers and controlmeans coupling said first and second variable displacement means suchthat the displacement of one of said motoring means is increased as theother of said motoring means is decreased.
 9. A fluid system comprising:a first tubular member having an internal bore enclosed at its oppositeends; a source of fluid pressure; a piston reciprocally mounted withinsaid bore and dividing said bore into two expansible pressure chambers;a piston rod carried on one side of said piston and extending throughone of said pressure chambers and externally of said tUbular member toone of said closed ends; said pressure chamber on the rod side of saidpiston being adapted to be communicated to said source of fluid pressureto generate a force against said piston to move same in one direction;the pressure chamber on the side of said piston opposite said rod beingadapted to be communicated to said source of fluid pressure to generatea force against said piston to move same in an opposite direction; asecond tubular member having an internal bore at one end, said secondtubular member having a longitudinal axis radially spaced from andparallel to the longitudinal axis of said first tubular member; a secondpiston reciprocably mounted within said second tubular member bore anddefining a second expansible pressure chamber therein; a piston rodcarried on one side of said second piston and extending through saidsecond pressure chamber and externally of said second tubular memberthrough said closed end; means for connecting the extended external endsof said piston rods such that said first and second pistons reciprocatetogether; first and second variable displacement motoring means, saidfirst motoring means having an inlet connected to the pressure chamberon the rod side of said first piston and an outlet connected to saidsecond pressure chamber, said second motoring means having an inletconnected to pressure chamber opposite said rod and an outlet connectedto said second pressure chamber, displacement control means couplingsaid first and second motoring means such that the displacement of oneof said motoring means is increased as the displacement of the other isdecreased; said second tubular member pressure chamber being adapted tobe selectively communicated to said source of pressure fluid throughsaid first and second motoring means to exert a force on the rod side ofsaid second piston to move said second piston in said one direction; andthe effective pressure responsive area of the side of said first pistonopposite said rod being substantially equal to the sum of the effectivepressure responsive area of the rod side of said first piston and theeffective pressure responsive area of said side of said second pistonassociated with said second tubular member pressure chamber.
 10. Thefluid system defined in claim 9 wherein said first and second motoringmeans comprise: a housing having a longitudinally disposed bore; a pairof axially spaced cylinder barrels rotatably mounted about a common axisin said housing bore, each of said cylinder barrels having a pluralityof arcuately spaced, parallel bores opening to the opposite faces ofeach of said cylinder barrels; first valving means at one end of saidhousing bore communicating one face of one of said cylinder barrels withhigh and low pressure passages defining said first motoring means inletand outlet; second valving means at the other end of said housing borecommunicating one face of the other of said cylinder barrels with highand low pressure passages defining said second motoring means inlet andoutlet; first piston means reciprocably mounted in said plurality ofbores in said one cylinder barrel and having outer ends extendingtherefrom for engagement with a thrust surface, the amount ofinclination of which controls the amount of displacement of said onecylinder barrel and thus the amount of displacement of said firstmotoring means; a second piston means reciprocably mounted in saidplurality of bores in said other cylinder barrel and having outer endsextending therefrom for engagement with a second thrust surface, theamount of inclination of which controls the displacement of said othercylinder barrel and thus the displacement of said second motoring means,said first and second thrust surfaces being formed on the opposite sideof a common swash plate adapted to be pivoted in one direction toincrease the displacement of one of said motoring means whilesimultaneously and in syNchronism decreasing the displacement of theother motoring means.